Sour Beer. While a certain level and quality of acidity is widely considered desirable in wine, often forming the backbone of its flavor structure, acidity is usually considered a flavor fault in modern beers. When speaking of beer, the word “sour” is usually pejorative. That said, there is a range of older beer styles that are traditionally acidic, and together with modern variants inspired by them, they have been termed, perhaps a bit rakishly, “sour beer.” When well made, they can be among the most complex and refreshing of beers, terrific with food and easily pushing the boundaries of what the modern drinker thinks of as “beer.”

In the days before modern sanitation, sour flavors in beer were common, if not always welcome. Most people avoided brewing during warm weather, when acidifying bacteria and wild yeast were more prevalent and likely to produce mouth-puckering acidity, sometimes with a vinegary tang, within days or weeks. When souring is intentional, the older “sour” beer styles are usually produced by the aging of beer in wooden barrels and involve the attempt to control acidity levels and produce an agreeable flavor. The classic sour beer styles include the German Berliner Weisse, and the Belgian Oud Bruin, Flanders Red, and lambic beers. All of these styles display a bracing lactic acidity, with the Belgian styles also showing some acetic character and a range of aromatics developed by bacteria and wild yeast during aging in oak barrels.

The rise of craft brewing in the United States and the emergence of upstart small breweries in other parts of the world has produced a new generation of brewers who aspire to create increasingly unique and flavorful beers. Many of these brewers have taken inspiration from classically sour Belgian beer styles. Not content with mimicking Belgian sour beers, they have started to develop what might be termed “new world” sour beers. Many of these new sour beers have no agreed-upon style guidelines and are yet to be classified in any particular category. This, of course, is part of the fun for the brewers who are making them. Some are aged in wine barrels, while others are aged in bourbon or whiskey barrels, successfully blending flavors that typically might not work well together. A handful of these “new world” brewers have even successfully created spontaneously fermented beers, something generally found only in Brussels or the Lambic region of Belgium. Some of these brewers have also installed “cool ships” in which to start the spontaneous fermentation before racking into oak barrels. Brewers who do not have the luxury of a cool ship have even used their mashing vessels as a temporary but suitable substitute in which to start their spontaneous fermentations. See coolship.

The wild yeast strain Brettanomyces is considered a scourge in most of the world’s vineyards, while being cautiously welcomed in others, especially among “natural” winemakers who employ no laboratory yeasts. Cultured Brettanomyces is often used in the making of new world sour beers. See brettanomyces. Desirable flavors often associated with sour beer using Brettanomyces are earthy, barnyard, mushroom, musty, and a general “funkiness.” Brettanomyces can be used with or without the introduction of bacteria, such as Lactobacillus and Pediococcus, during the barrel aging process. Because Pediococcus tends to produce the buttery-tasting compound diacetyl, Brettanomyces is often used in tandem to help minimize the undesirable characteristics that Pediococcus produces. In time, the Brettanomyces yeast will absorb the diacetyl, eventually eliminating unwanted buttery flavors.

Lactobacillus and Pediococcus are the two main bacteria that contribute to the actual acidity, or sourness, in the beer. Lactobacillus can ferment with or without oxygen and produces lactic acid and carbon dioxide, and yields a soft and mild tangy acidity. Fermenations using lactobacillus finish thinner and more cleanly on the palate than those using pediococcus. Conversely, pediococcus adds more rich and complex characteristics to the beer. Pediococcus ferments glucose into lactic acid, but it does not produce carbon dioxide. Skilled brewers of sour beer will blend these bacterial strains carefully, massaging fermentations to create the acid profile they desire in the beer. As individual barrels may progress differently, blending tends to be an essential part of the art of sour beer-making.

Enamored by the pungent flavors wild yeasts can create, some brewers have begun using Brettanomyces yeast for the entire primary fermentation, thus creating a 100% Brettanomyces fermented beer. These beers are generally produced in stainless steel tanks though some aging in oak barrels can make them even more interesting. In some cases, bacteria will be added, developing more mouthfeel and texture, thus adding to the overall complexity and structure of the beer. In this case, it is the addition of Lactobacillus and/or Pediococcus that will give the beer its acidity.

Belgium’s lambic brewers are content to have ambient microflora everywhere throughout the brewery, but that is not always the case among new world brewers of sour beer. Wild yeast strains and bacteria that are normally considered “spoilers” of wort and beer can be risky to use in a brewery producing “normal” beers with standard brewer’s yeasts; cross-contamination could be disastrous. As a result, most sour beer producers keep their “sour operations” as separate as possible from regular beer production, using different hoses, different bottle fillers, different tanks, and even separate buildings to keep the wild things at bay.

The development, largely by American craft brewers, of entire new categories of beer during the past decade, has resulted in the need for a new nomenclature to describe them. This nomenclature is surely unsettled, but the two terms in general use are “sour beer” and “wild beer.” “Wild beer” is generally used to describe any beer that displays the earthy characteristics of Brettanomyces yeast strains, regardless of whether the beer is a light golden ale or a strong dark stout. If the brewer adds acidifying bacteria to the beer, it is termed a “sour beer.” If both Brettanomyces character and bacterial acidity are in evidence, then the beer is generally deemed to fit both categories. Debates sometimes stretch into the wee hours, but with considerable humor, with brewers often lovingly referring to their wild yeasts and bacteria as “critters.” Regardless of how many angels eventually dance upon the head of this developing nomenclature, what is certain, if improbable, is that sour beers are taking hold, especially in the United States. Just as “natural winemaking” is slowly emerging from cult status, so is the production and enjoyment of sour beer, with some newly minted brewers focusing much of their energy into the development of a brave new world of beer flavor.

See also berliner weisse, flanders, lambic, oud bruin, and sourness.